Now that Montenegro's in the alliance, it could face the same pressure President Donald Trump has been putting on NATO members to pay 2 percent of their GDP to the organization.

Montenegro can't really make much of a financial contribution to the organization either way. Its fair share would only total about $80 million — lower than every other country's current rate of funding.

But the accession represents a symbolic snub to Russia, which has historic and cultural ties to the nation. Russia's Foreign Ministry said it "reserves the right to take counter-measures" to Montenegro's accession.